When a (human or animal) patient is perfused, it may be necessary to collect a sample from the patient, such as a blood sample, for analysis. In this case, an authorized person (doctor, nurse or veterinarian) performs:                either a sample collection independent of the perfusion. The patient is therefore subjected to a first needle insertion for fitting the perfusion and then a second needle insertion for the sample collection;        or a sample collection at the moment the catheter is put in place (or is changed), with a direct connection to the catheter, before putting the perfusion line in place;        or a sample collection, using a syringe, from the stopcock ramp of the extension (customarily a few tens of cm long) inserted between the perfusion tubing and the catheter. In this case, it is necessary to perform a first purge operation (suction of between 1 and 7.5 ml depending on the practices and the type of blood analysis to be carried out) then to carry out the blood sample collection itself. The reinjection of the purge suction is in general carried out quite rapidly into the extension in order to avoid a risk of hemolysis.        
The invention applies mainly, but not exclusively, to central or peripheral venous perfusions.
As is known, a perfusion in a peripheral vein is put in place in the following manner.
After having located a peripheral vein, the patient's skin is pierced using a needle covered with a catheter (such a device is known under the brand Cathlon®). Once the needle is positioned in a vein, the distal end of the catheter is pushed into the vein, the needle is removed, and the proximal end of the catheter is fastened to the patient's skin using a sterile dressing. Various diameters of catheter exist, and are generally expressed in terms of gauge (G). The diameter of the catheter used depends on the condition of the patient, on the operation to be carried out (sample collection or injection) and on the age of the patient. For example, the following catheters will be chosen: 22 to 24 gauge for newborns, 22 gauge for children from 1 month to 3 years old, 20 gauge for older children. For adults, it will be possible to choose catheters from 18 gauge (i.e. an external diameter of 1.1 mm to 1.3 mm) up to 14 gauge (i.e. an external diameter of 1.8 to 2.2 mm).
A table showing the connection between the “gauge” unit and the metric system is given below:
Gauge (internal diameterof the needle)External diameter (mm)24 G0.6 to 0.722 G0.7 to 0.920 G0.9 to 1.118 G1.1 to 1.316 G1.5 to 1.814 G1.8 to 2.2
The catheter bears, at its proximal end, a connection device, for example of Luer lock type, and at its distal end perfusion tubing. This connection device may be such that when the catheter is not connected, its proximal end is closed and the blood cannot escape.
In general, after connecting the catheter, a safety loop is produced with the tubing that is fastened to the patient's skin with tape. This safety loop prevents the catheter from being immediately pulled out in case of tension on the perfusion tubing.
The proximal end of the tubing is in fluid communication with an expansion vessel connected to a rigid or flexible bag of perfusion product attached to a pole. The latter must be high enough relative to the patient's catheter so that the perfusion product flows by simple gravity toward the catheter then the vascular system of the patient.
The tubing comprises, preferably, a toothed wheel for controlling the flow rate of perfusion, or other control system.
It is known to verify that the catheter is well positioned in the vascular system of the patient by detaching the perfusion bag from its pole and by lowering it to a height below that of the patient's catheter. Under these conditions, the blood pressure is greater than the pressure of the perfusion product. If the catheter is well positioned, it is possible to observe a reflux of blood into the perfusion tubing.
In order to avoid a second needle insertion into a perfused patient, document WO 2006/088771 proposes to equip a conventional perfusion with a reversible pump associated with control means. The latter is designed to intermittently interrupt the operation of the perfusion pump in the forward direction (that is to say from the bag of perfusion product toward the patient), in order to operate the pump in the rearward direction (that is to say from the patient to a sample collection circuit). In this way, it is possible to collect a sample of the patient's blood via the perfusion catheter.
However, this device requires meticulous control of the pump and a complex fluid circuit comprising multiple valves.
Moreover, this system may be noisy (due to the operation of the pump) and requires a source of energy for the collection of a sample of blood and the injection of the perfusion product.